Notice and error when trying to import block

To reproduce

- install two drupal 8 sites, source and destination, with the standard install profile.
- copy plugin.core.block.bartik.content from /source/sites/default/files/config_*/active to /destination/sites/default/files/config_*/staging
- Go to http://destination/admin/config/development/sync and click import all(*)
- You will get a notice and the import won't succeed: Notice: Undefined index: block in Drupal\Core\Config\Entity\ConfigStorageController->importChange() (line 465 of core/lib/Drupal/Core/Config/Entity/ConfigStorageController.php).
- Click Import all again
- Now you'll get an error: Fatal error: __clone method called on non-object in /Users/tp1/Sites/drupal8dest/core/lib/Drupal/Core/Config/Entity/ConfigStorageController.php on line 466

(*) Drupal sees the imported block as different from the existing block because its UUID is different (although it is identical in all other respects).